(a) Milk Bottle. (b) Pipet. (c) Acid Measure. (d), (e), (f) Different varieties of Centrifugal Machines. obliquely while doing so. (N. B., remember that sulphuric acid is very corrosive.) Whirl the bottle in the centrifugal machine for five minutes. Remove the bottle, add enough hot water to bring the fat into the narrow neck. The fat can be easily discerned, for it is lighter in color than the rest of the liquid, which holds the decomposed organic matter in solution. After the addition of the water, replace the bottle in the centrifugal machine and whirl it for three minutes. This brings all the fat to the top. Then remove the bottle and, holding it with the fat on a level with the eye, note the marks opposite its upper and lower levels. To estimate the per cent. of fat it is necessary to subtract the number below the level of the fat from the top reading. For example, if the upper level of the fat is on a line with the mark 5.2 and the lower level with the mark 1. 2, the correct reading will be 4 per cent. since 5.2-1.2=4. This would show that the milk tested contained 4 per cent. fat. CHAPTER XXII CHEMISTRY OF DIGESTION The Nature of the Changes Occurring in the Digestion of the Various Food Stuffs-The Organs in Which These Changes Occur-The Factors and Conditions Influencing Digestion -Nature of Enzymes, Zymogens, and Kinases. Reason for digestion.-With the exception of the monosaccharids and the disaccharids, organic food substances are unable to pass through animal membranes and, consequently, they are of no use to the body until they undergo certain changes which transform them into substances that will go into solution and pass through the walls of the blood-vessels and lacteals. For it is only after food has passed into the blood and been carried by it to the tissues that it benefits the body. The various changes that food undergoes in this process of preparation are classed as digestion. Mineral matter, monosaccharids, and disaccharids are readily soluble in water and the two first mentioned are quickly absorbed from the stomach and the intestines. Disaccharids will be absorbed unchanged if eaten in large quantities, but, when this happens, they are quickly eliminated from the system in the urine, the tissues being unable to utilize them. Nature of digestion.-The digestion of proteins and carbohydrates consists of a process of hydrolysis-i. e., decomposition due to the absorption of, and chemical combination with, water. The digestion of fats consists of a preliminary splitting of the fats into fatty acids and glycerin, followed by the saponification of the fatty acids. Results of hydrolysis and saponification.-As the result of hydrolysis, complex molecules are divided and simpler substances thus formed; for example, one molecule of a disaccharid (C12H22O11) by hydrolysis gives 2 molecules of a monosaccharid 2 (C6H12O6). There is only one splitting of the molecules in the hydrolysis of the disaccharids, but the polysaccharids (CHO) pass through several stages before they are completely hydrolyzed. The substances yielded at the different stages in the digestion of starch are dextrins (known, according to the colors which they give with iodin, as erythrodextrin 1, 2, or 3 and achrodextrin, see page 398), maltose, and glucose. The protein molecule also undergoes several splittings, thereby giving rise to such substances as metaproteins, proteoses, peptones, and amino acids. The nature and the results of saponification whereby fats are digested have been already discussed (Chapter XVI.). How hydrolysis and saponification can be brought about.-Starches, sucroses, and proteins can be hydrolyzed and thereby made to undergo the changes described in the preceding paragraph by boiling them with acid and by the use of ferments. The ferments used to digest starch outside the body are usually obtained from plants; one of the best known is diastase of malt, a substance that is produced during the ger The x, it will be remembered, signifies that the molecules of polysaccharids contain an unknown number of molecules with the construction demonstrated by the formula. mination of certain seeds and that is contained in malt. The ferments used to digest proteins are extracted from the stomach or pancreas of animals, especially pigs. Within the body, hydrolysis is caused by the ferments or enzymes contained in the digestive juices. Saponification, outside the body, is brought about by boiling fats with alkalies and, within the body, by an enzyme contained in the pancreatic juice and the alkaline salts of the pancreatic and intestinal juices and the bile. Nature of the digestive juices.-The digestive juices secreted by the salivary glands, by the gastric and intestinal glands, and the pancreas consist of water, salts, mucin, and specific enzymes. The reaction of the saliva is, normally, neutral or faintly alkaline; that of the gastric juice, acid (due to the presence of hydrochloric acid); and that of the pancreatic and intestinal juices, alkaline. The bile, which is secreted from the blood by the liver, consists of water, bile salts, and inorganic salts, mucin, cholesterin, lecithin, fat, and bile pigments. The bile pigments are derived from hemoglobin; it is thought that when red blood-corpuscles become disintegrated the freed hemoglobin is brought to the liver, where, under the influence of the liver cells, its iron is split off and it is converted into bilirubin or biliverdin, the characteristic pigments or coloring matter of bile. Normally, bile has an alkaline reaction. Enzymes, Zymogens, Kinases, Hormones The nature and action of these substances which are so essential to digestion and metabolism are as yet very imperfectly understood. |